Nobody will ever confuse a dancebelt with silk boxers.  Most guys will tell you it takes awhile to get used to the unique tight feeling you get in a dancebelt.  Realistically, your boy has never worn anything even half as tight and he is going through many pyschological and physical reactions to it.  Teens typically dismiss anything that draws into question their imperviousness to pain and basic claim to immortality.  Dancebelts deal with a teenager's sex organs in a pretty specific and unyielding way that is embarrassing for many of them to talk about.

However, dancebelts were invented for a reason - not just to cramp your style.  Your kid has already had one dose of pain caused by not wearing one.  How many dose like that does he need?

It's also possible you bought him a dancebelt that really doesn't fit and will never be comfortable.  You might ask him if he'd like to try another brand or size.  Bottom line, insist he wear one.